CO2-Responsive Pickering Emulsions Stabilized by a Bio-based Rigid

Sep 26, 2018 - Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University. No. 159 Longpan Road, Xu...
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Article Cite This: J. Agric. Food Chem. 2018, 66, 10769−10776

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CO2‑Responsive Pickering Emulsions Stabilized by a Bio-based Rigid Surfactant with Nanosilica Xinyan Yan,† Zhaolan Zhai,† Ji Xu,† Zhanqian Song,† Shibin Shang,†,§ and Xiaoping Rao*,†,‡,§

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Institute of Chemical Industry of Forest Products, CAF, National Engineering Lab. for Biomass Chemical Utilization; Key and Open Lab. of Forest Chemical Engineering, SFA; Key Lab. of Biomass Energy and Material. No. 16 Suojinbei Road, Xuanwu District, Nanjing, Jiangsu Province 210000, China ‡ Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University. No. 159 Longpan Road, Xuanwu District, Nanjing, Jiangsu Province 210000, China § Research Institute of Forestry New Technology, CAF, No. 1 Xiangshan Road, Haidian District, Beijing, 100091, China S Supporting Information *

ABSTRACT: A novel CO2-responsive surfactant, maleopimaric acid glycidyl methacrylate ester 3-(dimethylamino)propylamine imide (MPAGN), based on sustainable resource of rosin was synthesized and used to prepare a kind of CO2responsive Pickering emulsions with nanosilica. MPAGN can be reversibly responsive to CO2 and N2 between active cationic (MPAGNH+) and inactive nonionic (MPAGN), leading to adsorb on or desorb from the surface of nanosilica, then stabilize or break emulsion. CO2-responsive behavior of MPAGN was verified by cycle change of pH and conductivity with bubbling CO2 and N2 alternately. The type of adsorption of MPAGNH+ at the particle−water interface was explained according to the adsorption isotherms. The mechanisms of stabilization, destabilization, and restabilization of Pickering emulsion were analyzed according to zeta potentials and droplet size. This Pickering emulsion can be reversible between stable and unstable by bubbling CO2 and N2 alternately. Moreover, this emulsifier can be recycled when new oil was added after removing the initial oil. Therefore, it not only has economic benefits but also has an environmentally friendly property. KEYWORDS: rosin, rigid, CO2-responsive, Pickering emulsions, recycled

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INTRODUCTION Conventional emulsions are stabilized by amphiphilic polymers or surfactants,1 and they are thermodynamically instable systems resulting in coalescence, sedimentation, creaming, or phase inversion.2 A far higher concentration than critical micelle concentration (CMC) is usually needed to form a stable emulsion. Pickering emulsion can be very stable and only required a little amount of surfactant because it is stabilized by surfaceactive colloid particles, which provide a mechanical barrier to cream.3 However, it is difficult to be demulsified when it is used in some areas such as drug delivery,4 emulsion polymerization,5 oil transport, and fuel production.6 Therefore, it has become a crucial focus how to balance between stability and instability of a Pickering emulsion. Responsive surfactants can be switched reversibly between inactive and active under the control of trigger. Responsive surfactants can be classified into two broad categories: one responds to the electrochemical stimuli such as pH,7−13 CO2,14−20 redox,21,22 etc., and the other responds to the physical stimuli, such as light,23−26 temperature,27 magnetic field,28−30 etc. The responsive surfactant can be transferred to charged particles by electrostatic interaction, and the combination can be used to prepare responsive Pickering emulsion, which can exist for a long time while it can demulsify immediately upon exposure to a stimulus. As a biocompatible, nontoxic, inexpensive, and easily removable trigger, CO2 has attracted more attention from researchers. There are two © 2018 American Chemical Society

methods commonly used to prepare CO2-responsive Pickering emulsion. One is to use CO2-responsive particles, which are usually polymeric particles. However, these particles generally require complex synthetic procedures.31 The other is to use a CO2-responsive compound, which was adsorbed on the surface of nanoparticles by electrostatic interaction. The latter was better than the former as the performance of the particles could be controlled by changing the concentration or structure of the compound. Liu and co-workers32 first reported in 2006 that long-chain alkyl amidine could be switched by bubbling CO2 and N2 and can be used as switchable emulsifier. Jiang33 reported a responsive Pickering emulsion stabilized by existing responsive surfactant (N′-dodecyl-N,N-dimethylacetamidine) with nanosilica particle. Zhang and co-workers31 fabricated a CO2-responsive Pickering emulsion based on long hydrophobic tertiary amine with nanosilica particle. They also reported a CO2-responsive Pickering emulsion stabilized by amine oxide-based surfactant (C14PAO) with nanosilica particle.34 In these reports, the materials of CO2 responsive surfactants were often restricted to existing long-chain alkane surfactants, which were mainly derived from petrochemical resources. Received: Revised: Accepted: Published: 10769

July 2, 2018 August 30, 2018 September 26, 2018 September 26, 2018 DOI: 10.1021/acs.jafc.8b03458 J. Agric. Food Chem. 2018, 66, 10769−10776

Article

Journal of Agricultural and Food Chemistry Scheme 1. Synthesis Route of MPAGN

Synthesis of MPAGN. Synthesis of Maleopimaric Acid (MPA). Four hundred grams of rosin and 140 g of maleic acid were dissolved in 160 g of glacial acetic acid. The mixtures were stirred at 140 °C for 4 h. After cooling, the mixtures were poured into 400 g of glacial acetic acid, and the crystalline MPA was obtained after suction filtering and dryer. The crystal was further purified by crystallization using acetic acid for another two times41 (purity: 93%). Synthesis of Maleopimaric Acid Glycidyl Methacrylate Ester (MPAG). Forty grams of MPA and 15.6 g of GMA were dissolved into 210 g of toluene. Triethylamine (0.5 wt % with respect to MPA) was used as the catalyst, and hydroquinone (0.2 wt % with respect to GMA) was used as inhibitor. The mixtures were stirred at 120 °C for 8 h. Then, the toluene was evaporated. Synthesis of Maleopimaric Acid Glycidyl Methacrylate Ester 3(dimethylamino)propylamine Imide (MPAGN). Obtained MPAG was dissolved in 200 g of ethyl alcohol and stirred at 85 °C. Then, a solution of 11.2 g of 3-(dimethylamino)propylamine dissolved in 10 g of ethyl alcohol was dropped in the flask. They were stirred at 85 °C for 5 h. The crude product was purified via column chromatography (the ratio of methyl alcohol to dichloromethane = 1:9) to obtain the target product MPAGN (yield: 24%). The synthesis route of MPAGN was shown in Scheme 1. This type of surfactant was polymerizable. Therefore, it maybe has potential application value in the emulsifier free emulsion polymerization field. First, in this study, its emulsifying property was studied. Preparing of Pickering Emulsions. MPAGN was protonated and formed cationic tertiary amine bicarbonate by bubbling CO2. To distinguish the cationic tertiary amine bicarbonate from the nonionic form of MPAGN, the former was written as MPAGNH+. Nanosilica particles (0.5 wt %) were dispersed in different concentrations of MPAGNH+ using an ultrasonic cell disrupter (750 W and 20 kHz) for 1 min. Seven milliliters of MPAGNH+ solution, or nanosilica dispersion, or MPAGNH+ with nanosilica and 7 mL of liquid paraffin were put in a 25 cm3 glass bottle followed by homogenization at 11000 rpm for 5 min (IKA T18, S25N-8G head). The concentrations of MPAGNH+ and nanosilica reported in this article were expressed as mmol per liter (mM), and weight percent (wt %) referred to the aqueous phase.6 The emulsions were stored for at least 24 h at 25 °C before being analyzed. Measurements. Conductivity/pH. The conductivity/pH of MPAGN solutions with bubbling CO2 or N2 were measured by a DDS-307A conductometer and Thermo Scientific Orion pH meter at 25 °C, respectively. The gas flow rate of CO2 or N2 was about 100 mL/min. Surface Tension. The surface tensions of different concentrations of MPAGNH+ solution and with nanosilica were measured by a

Studies on sustainable chemistry have drawn attention widely because of the urgency for resolving environmental problems.35 Rosin is a kind of unique sustainable biomass resource, obtained from pine trees. It has a unique tricyclic rigid structure,36 and its derivatives are abundant, renewable, sustainable, economical, and nontoxic.37 It can be used to prepare surfactants thanks to the excellent hydrophobicity of its tricyclic rigid backbone.38 We previously synthesized some surfactants based on rosin and studied their properties such as aggregation behavior,39 emulsifying properties,38 and dispersing properties.40 In this study, a novel CO2-responsive surfactant based on natural tricyclic rosin was synthesized, which has a lower critical micelle concentration (CMC) than that of long-chain alkane CTAB, thanks to its hydrophobic tricyclic hydrophenanthrene rigid skeleton. This surfactant can be switched reversibly between active cationic (water soluble) and inactive nonionic (water insoluble) by bubbling CO2 and N2. A CO2responsive Pickering emulsion was prepared, which was stabilized by this CO2-responsive surfactant in combination with 0.5 wt % nanosilica. This Pickering emulsion can be switched many times between stable and unstable by bubbling CO2 and N2. Even more, after demulsification by bubbling N2, the emulsifier can be recycled so it embodies the ideals of green and sustainable development in the field of marketing.

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EXPERIMENTAL SECTION

Materials and Equipment. Rosin was received from Westech Chemical Co. Ltd. (China). Nanosilica (purity >99.5%, primary particle diameter ≈ 30 nm; BET surface area ≈ 115 m2/g) and glycidyl methacrylate (GMA) were obtained from Aldrich (China). 3(Dimethylamino)propylamine was obtained from Energy Chemical (China). Hydroquinol, triethylamine, maleic acid, glacial acetic acid, absolute ethyl alcohol, absolute methyl alcohol, and dichloromethane were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Methylbenzene was obtained from Nanjing Chemical Reagent Co. Ltd. (China). They were all analytically pure and used as received. Distilled water was produced by Hitech laboratory purification system (with a pH of 6.1 and electrical resistivity of 18.2 MΩ·cm). FT-IR spectra were recorded on a Thermo Scientific Nicolet IS10. 1 H NMR was measured by an Advance III 400 MHz spectrometer (Bruker, Germany). Mass spectrum was performed on an Agilent5973 spectrometer (Agilent Technologies, USA). 10770

DOI: 10.1021/acs.jafc.8b03458 J. Agric. Food Chem. 2018, 66, 10769−10776

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Journal of Agricultural and Food Chemistry

Scheme 2. Reversible Transformation between Cationic MPAGNH+ and Nonionic MPAGN; Inset Photographs Correspond to the Appearance of Nonionic and Cationic Forms in Water

Sigma701 at 25 °C. The CMC and surface tension (γ) of MPAGNH+ solution were obtained when the adsorption balance was reached. They were equilibrated at 25 °C for at least 12 h before measurement. Zeta Potentials. The zeta potentials of 0.5% nanosilica particles dispersed in different concentrations of MPAGNH+ solution were measured by a zeta sizer instrument (Nano ZS ZEN 3600, Malvern Instruments Ltd.). They were equilibrated at 25 °C for at least 12 h before being analyzed. Adsorption of MPAGNH+ at Silica−Water Interface. The adsorption isotherm of MPAGNH+ at the interface of particle− water at equilibrium concentration (a∞), implying that it was the monolayer between adsorption particle surface and MPAGNH+. a0 a= NA Γs/w (4)

Figure 5. Surface tension of MPAGNH+ solutions with 0.5 wt % nanosilica, and adsorption isotherm of MPAGNH+ at the particle− water interface.

hydrophobic tails N-(3-(dimethylamino)propyl) alkyl amide (C14PMA, 2.35 mM),30 which indicated that cationic surfactant MPAGNH+ had excellent surface activity. At equilibrium concentration less than CMC, the adsorption isotherm of MPAGNH+ on the interface of silica−water was measured according to the depletion method at 25 °C. At the presence of particles, the surface tension of solution would not be affected, so they need not be separated. However, the surfactant in solution would be adsorbed on the surface of silica, resulting in the equilibrium concentration of surfactant in solution reducing and the surface tension increasing. Therefore, the adsorbed amount can be calculated based on the difference between the surface tension of MPAGNH+ solution alone and with nanosilica. The curves of log C−surface tension (γ) of different concentrations of MPAGNH+ solution with and without nanosilica were shown in Figure 5, and that without nanosilica was fitted by Origin 8.5 followed by the Szyszkowski equation, eq 1. γ0 − γ = nRT Γ∞ ln(1 + KC)

V (C 0 − C ) w

where a was the average area molecular on the surface of nanosilica; a0 was the average surface area of the nanosilica; NA was Avogadro’s number; Γs/w was the adsorption amount of MPAGNH+ on the particle−water interface. The 0.5 wt % nanosilica was negatively charged in pure water (−37.9 mV). When it was dispersed in MPAGNH+ solution, the zeta potential of dispersion increased to positive with concentration of MPAGNH+ increased as shown in Figure 6. It can be concluded that the adsorption between cationic MPAGNH+ and silica occurred, forming a monolayer via electrostatic interactions. Destabilization/Restabilization Cycling of This Pickering Emulsion. CO2/N2 responsive behavior of MPAGN has been confirmed in the above discussion. So, it was predicted that the Pickering emulsion stabilized by MPAGNcoated particles may be also CO2/N2 responsive and can be cycled between stable and unstable by bubbling CO2 and N2

(1)

where γ0 was the surface tension of distilled water (72 mN/m); γ was the surface tension of MPAGNH+ solution with nanosilica (mN/m); R was the gas constant (8.314 × 107 erg/(mol K)); T was the Kelvin temperature (298.15 K); Γ∞ was the maximum adsorption amount of surfactant at gas liquid interface (mol/cm2); K was constant (L/mol); and C was equilibrium concentration of surfactant (mol/L). After fitting, as shown in Figure S5, the parameters Γ∞ and k were obtained. They are 1.24 × 10−10 mol/cm2 and 29247 L/mol, respectively, and the correlation coefficient R2 was about 0.96. The cross-sectional area at the water−air interface adsorption (a∞) was 1.34 nm2 calculated according to eq 2. 1 a∞ = NA Γ∞ (2) where a∞ was the cross-sectional area at the water−air interface of adsorption when the amount of adsorbed surfactant was maximum (mmol/g); Γ∞ was the maximum adsorbed amount of surfactant at gas−liquid interface (mol/ cm2); and NA was Avogadro’s number. The equilibrium concentration of surfactant in dispersion was known based on the surface tension of MPAGNH+ solution with nanosilica

Figure 6. Zeta potential of 0.5 wt % nanosilica dispersed in different concentrations of MPAGNH+ solution; (inset) graph of cycles of 0.5 wt % nanosilica dispersed in 0.4 mM MPAGNH+ solution. 10773

DOI: 10.1021/acs.jafc.8b03458 J. Agric. Food Chem. 2018, 66, 10769−10776

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Journal of Agricultural and Food Chemistry alternately. To prove this assumption, a liquid paraffin-in-water emulsion stabilized by 0.05 mM MPAGNH+ and 0.5 wt % nanosilica was cycled by bubbling CO 2 followed by homogenization and then bubbling N2. In cycle 1, a stable emulsion was formed by bubbling CO2 at 25 °C for only 3 min followed by homogenization. However, when N2 was bubbled to replace CO2 at 60 °C for 30 min, the stable Pickering emulsion was demulsified completely as shown in Figure 7, and

Figure 8. Photographs of Pickering emulsion stabilized by 0.1 mM MPAGNH+ and 0.5 wt % nanosilica in which initial oil was replaced by the new one.

Figure 7. Photographs and size distribution of Pickering emulsion stabilized by 0.05 mM MPAGNH+ and 0.5 wt % nanosilica with CO2/ N2 cycles. Figure 9. Size distribution of Pickering emulsion stabilized by 0.05 mM MPAGNH+ and 0.5 wt % nanosilica with initial oil and new oil.

it cannot form stable emulsion even though rehomogenization was performed. To rule out the effect of temperature on the demulsification, the emulsion did not appear demulsified and remained stable as shown in Figure S6 when it was heated at 60 °C for 30 min without bubbling N2, which indicated that N2 was the key factor rather than temperature in the process of demulsification. Interestingly, stable emulsion was formed once more when bubbling CO2 through solution again under the same condition as the above-described. The size distribution had little change between initial emulsion and the second emulsion after response as shown in Figure 7, while the droplet size would present an obvious increase if pH was triggered by adding HCl/NaOH alternately because of the accumulation of NaCl.46 The emulsion would be unstable in the presence of electrolyte.47 In conclusion, CO2 as a trigger in Pickering emulsion had a lot of advantages compared to the pH trigger because the composition of the system did not change after bubbling CO2 and N2 alternately. After demulsification, could the emulsifier be recovered and reused? Based on this doubt, the oil phase of completely demulsified Pickering emulsion was removed, and the residual water phase was collected. Seven milliliters of new liquid paraffin was added into the collected water phase followed by homogenization at 11000 rpm for 5 min, and a new Pickering emulsion formed. The operational processes were shown in Figure 8. After this cycle, the size distribution had little change between liquid paraffin-in-water Pickering emulsion stabilized by 0.05 mM MPAGNH+ and 0.5 wt % nanosilica with initial oil phase and the Pickering emulsion with new oil phase as shown in Figure 9. After emulsion worked, the emulsifier can be recycled; so it not only has economic benefits but also has environmentally friendly properties.

Mechanism of Destabilization and Restabilization of Emulsions. In the presence of CO2, the nanosilica was hydrophobized by the adsorption of the MPAGNH+ molecules by electrostatic interaction between cationic surfactant and anionic particles. However, when N2 was bubbled into the system to expel CO2, cationic surfactant was reversed back to the nonionic form and the electrostatic interaction disappeared. Therefore, the coated particle was recovered to the original bare form again, and demulsification happened. When CO2 was bubbled into system again, the MPAGN was switched to the cationic form MPAGNH+ and adsorbed on the surface of the particle once again. This supposition can be proved by the transformation of zeta potential of the nanosilica dispersed in MPAGNH+ solution with CO2/N2 cycles. The zeta potential was −37.9 mV when the nanosilica was dispersed in water, but it increased to −8.5 mV when dispersed in 0.4 mM MPAGNH+, implying MPAGNH+ has adsorbed on the nanosilica by electrostatic interaction in this process. After bubbling N2, the zeta potential of dispersion was back to −37.9 mV. implying that the coated particle was recovered to the original bare form, and the electrostatic interaction disappeared between cationic surfactant and anionic particles. This cycle can be repeated for several times as shown in the inset graph of Figure 7 In summary, a novel, rigid CO2-responsive surfactant based on natural forestry resources rosin MPAGN was reported for the first time, and its CMC was 0.4 mM lower than that of a conventional cationic surfactant CTAB (0.9 mM) and lower than that of a tertiary amine with long hydrophobic tails C14PMA (2.35 mM). Upon bubbling CO2 into the system, the 10774

DOI: 10.1021/acs.jafc.8b03458 J. Agric. Food Chem. 2018, 66, 10769−10776

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Journal of Agricultural and Food Chemistry

in situ with a conventional cationic surfactant. Langmuir 2015, 31 (11), 3301−3307. (4) Fernandes, D. A.; Fernandes, D. D.; Li, Y.; Wang, Y.; Zhang, Z.; Rousseau, D.; Gradinaru, C. C.; Kolios, M. C. Synthesis of Stable Multifunctional Perfluorocarbon Nanoemulsions for Cancer Therapy and Imaging. Langmuir 2016, 32 (42), 10870−10880. (5) Schrade, A.; Landfester, K.; Ziener, U. Pickering-type stabilized nanoparticles by heterophase polymerization. Chem. Soc. Rev. 2013, 42 (16), 6823−6839. (6) Zhu, Y.; Jiang, J.; Liu, K.; Cui, Z.; Binks, B. P. Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Conventional Cationic Surfactant. Langmuir 2015, 31 (11), 3301−3307. (7) Liu, H.; Wang, C.; Zou, S.; Wei, Z.; Tong, Z. Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification. Langmuir 2012, 28 (30), 11017− 11024. (8) Fujii, S.; Mochizuki, M.; Aono, K.; Hamasaki, S.; Murakami, R.; Nakamura, Y. pH-responsive aqueous foams stabilized by hairy latex particles. Langmuir 2011, 27 (21), 12902−12909. (9) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S.; Schmid, A. pH-responsive aqueous foams stabilized by ionizable latex particles. Langmuir 2007, 23 (17), 8691−8694. (10) Shu, X.; Meng, Y.; Wan, L.; Li, G.; Yang, M.; Jin, W. pHResponsive Aqueous Foams of Oleic Acid/Oleate Solution. J. Dispersion Sci. Technol. 2014, 35 (2), 293−300. (11) Lin, Q.; Liu, K.; Cui, Z.; Pei, X.; Jiang, J.; Song, B. pHResponsive Pickering Foams Stabilized by Silica Nanoparticles in Combination with Trace Amount of Dodecyl Dimethyl Carboxyl Betaine. Colloids Surf., A 2018, 544, 44. (12) Fujii, S.; Akiyama, K.; Nakayama, S.; Hamasaki, S.; Yusa, S.; Nakamura, Y. pH- and temperature-responsive aqueous foams stabilized by hairy latex particles. Soft Matter 2014, 11 (3), 572−579. (13) Li, J.; Stöver, H. D. H. Doubly pH-Responsive Pickering Emulsion. Langmuir 2008, 24 (23), 13237−13240. (14) Liang, C.; Liu, Q.; Xu, Z. Surfactant-free switchable emulsions using CO2-responsive particles. ACS Appl. Mater. Interfaces 2014, 6 (9), 6898. (15) Su, X.; Jessop, P. G.; Cunningham, M. F. Surfactant-Free Polymerization Forming Switchable Latexes That Can Be Aggregated and Redispersed by CO2 Removal and Then Readdition. Macromolecules 2012, 45 (2), 666−670. (16) Yan, Q.; Zhou, R.; Fu, C.; Zhang, H.; Yin, Y.; Yuan, J. CO2responsive polymeric vesicles that breathe. Angew. Chem., Int. Ed. 2011, 50 (21), 4923−4927. (17) Lin, S.; Theato, P. CO2-Responsive Polymers. Macromol. Rapid Commun. 2013, 34 (14), 1118−1133. (18) Zippin, J. H.; Levin, L. R.; Buck, J. CO2/HCO3−-responsive soluble adenylyl cyclase as a putative metabolic sensor. Trends Endocrinol. Metab. 2001, 12 (8), 366−370. (19) Zhang, Q.; Yu, G.; Wang, W. J.; Yuan, H.; Li, B. G.; Zhu, S. Preparation of N2/CO2 Triggered Reversibly Coagulatable and Redispersible Latexes by Emulsion Polymerization of Styrene with a Reactive Switchable Surfactant. Langmuir 2012, 28 (14), 5940. (20) Zhang, Q.; Yu, G.; Wang, W. J.; Li, B. G.; Zhu, S. Preparation of COa/Na-triggered reversibly coagulatable and redispersible polyacrylate latexes by emulsion polymerization using a polymeric surfactant. Macromol. Rapid Commun. 2012, 33 (10), 916−921. (21) Quesada, M.; Muniesa, C.; Botella, P. Hybrid PLGAOrganosilica Nanoparticles with Redox-Sensitive Molecular Gates. Chem. Mater. 2013, 25 (13), 2597−2602. (22) Zhang, Y.; Guo, S.; Ren, X.; Liu, X.; Fang, Y. CO2 and Redox Dual Responsive Pickering Emulsion. Langmuir 2017, 33 (45), 12973. (23) Anwar, N.; Willms, T.; Grimme, B. Kuehne AJC: LightSwitchable and Monodisperse Conjugated Polymer Particles. ACS Macro Lett. 2013, 2 (9), 766−769. (24) Asshoff, S. J., Lancia, F., Sukas, S., Blonde, J. B., Yamaguchi, T., Hommersom, C. A., Gac, S. L., Katsonis, N. Photo-switchable Distributors of Light Based on Self-Assembled Monodisperse Chiral

nonionic MPAGN was reversed to the active cationic form, and the cationic was adsorbed on the surface of negatively charged particles via electrostatic interactions forming a monolayer. The coated particles were used to prepare a paraffin-in-water Pickering emulsion. This kind of Pickering emulsion can exist stably for a long time. After bubbling N2, cationic quaternary ammonium was deprotonated and recovered to an inactive nonionic tertiary amine molecule, so the electrostatic interaction between surfactant and particles disappeared, leading to desorption from the surface of nanosilica, and demulsification occurred immediately. Interestingly, stable emulsion was formed again when CO2 was bubbled into the system again under the same condition as the last time. This Pickering emulsion can be reversible between emulsification and demulsification with bubbling CO2/N2. Moreover, this emulsifier can be recycled when new oil was added after removing the initial oil. Therefore, it not only has economic benefits but also has environmentally friendly properties. Besides, this type of CO2-responsive surfactant not only possesses good emulsifying performance but also was polymerizable. Therefore, it maybe has potential application in the emulsifier-free emulsion polymerization field. We will explore further in our next work.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.8b03458. Characterization of molecules, FT-IR, 1H NMR and MS spectra, photographs of a drop of emulsion dispersed into water and into oil, surface tension of MPAGN solution without SiO2 as a function of concentration of MPAGN at 25 °C and fit line, photographs of liquid paraffin-in-water emulsion stabilized by 0.05 mM MPAGN and 0.5 wt % nanosilica at 60 °C for 30 min without N2 are shown in Figures S1−S6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: + 86 25-85413445. Tel: + 86-25-85482452. ORCID

Shibin Shang: 0000-0002-8665-769X Xiaoping Rao: 0000-0001-5115-8260 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by grants from National Natural Science Foundation of China (31622017 and 31470596) and Fundamental Research Foundation of the Central Commonwealth Institute of the Chinese Academy of Forestry (CAFYBB2018QC002).

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REFERENCES

(1) Liu, K.; Jiang, J.; Cui, Z.; Binks, B. P. pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant. Langmuir 2017, 33 (9), 2296. (2) Mcclements, D. J. Food Emulsions: Principles, Practice, and Techniques; CRC Press, 1998. (3) Zhu, Y.; Jiang, J.; Liu, K.; Cui, Z.; Binks, B. P. Switchable Pickering emulsions stabilized by silica nanoparticles hydrophobized 10775

DOI: 10.1021/acs.jafc.8b03458 J. Agric. Food Chem. 2018, 66, 10769−10776

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DOI: 10.1021/acs.jafc.8b03458 J. Agric. Food Chem. 2018, 66, 10769−10776